1. Field of the Invention
The present invention relates to a method of decreasing an amount of harmful oxide in an exhaust gas generated from a fluidized bed combustor for combusting solid hydrocarbon fuel, and more specifically, to a method of controlling an amount of a treatment agent to be injected into an exhaust gas and an apparatus therefor.
2. Description of the Related Art
Conventionally, a method of controlling an amount of harmful oxide such as nitrogen oxide (hereinafter, referred to as NOx) in an exhaust gas generated from a combustor includes catalytic reduction which uses a reductive catalyst and ammonia NH.sub.3 as a reductant and selective non-catalytic reduction which injects a reductant into a combustor without using a catalyst. The catalytic reduction measures nitrogen oxide NOx in an exhaust gas upstream of a position at which the reductant NH.sub.3 is supplied by a NOx analyzer and the reductant is supplied while controlling the amount thereof according to the measured value so that a predetermined mole ratio is obtained with respect to NOx. On the other hand, the selective non-catalytic reduction also measures NOx in an exhaust gas from the combustor by the NOx analyzer and injects the reductant while controlling the amount thereof according to the measured value so that a predetermined mole ratio is obtained with respect to NOx.
However, when the selective non-catalytic reduction of the above conventional technologies is to be applied to a fluidized bed pressurized boiler, the following problems arise. That is, a residence time of a gas, which is required by a reaction time during which NH.sub.3 decreases an amount of NOx by reducing it to a target value, must be secured, and for this purpose it is necessary to inject NH.sub.3 upstream of a cyclone installed downstream of the fluidized bed pressurized boiler and to proceed a reducing reaction in a high temperature pipe. However, since particles are scattered in an exhaust gas in the above method, a cyclone of any form must be installed upstream of the inlet of the NOx analyzer and further maintenance and inspection of the cyclone is necessary to operate the boiler for a long time. Further, when there is provided a sampling port which branches from the high temperature pipe connecting the outlet of the furnace of the fluidized bed pressurized boiler to the cyclone, a problem arises as to the strength of the high temperature pipe having a high internal pressure. Consequently, it is desired to calculate an amount of harmful oxide in an exhaust gas from a calculation formula including parameters such as amounts of fuel and air supplied which are controllable relating to combustion, a bed height and the like and to inject a treatment agent in an amount optimum to the calculated amount of harmful oxide into the exhaust gas and decrease harmful oxide to the target amount thereof without measuring NOx in the exhaust gas by a NOx analyzer.